1. Field of the Invention
The present invention relates to a process for producing a crystalline silicon nitride powder which is useful as a starting material for producing a sintered silicon nitride material suitable for high temperature structural applications.
2. Description of Related Art
A conventional method of producing a crystalline silicon nitride powder by baking an amorphous silicon nitride powder produced by thermal decomposition of a nitrogen-containing silane compound, for example, silicon diimide, in an inert gas atmosphere is known.
In this conventional method, to enhance the sintering property of the resultant crystalline silicon nitride powder, it is necessary that the specific surface area and oxygen content of the powder are respectively in an appropriate range.
Also, there is a close relationship between the specific surface area and the oxygen content of the crystalline silicon nitride powder. Namely, it is known that the oxygen content increases with increase in the specific surface area, and the specific surface area increases with increase in the oxygen content. This is because oxide layers or oxynitride layers form on the surfaces of the crystalline silicon nitride particles.
Accordingly, to control the specific surface area of the crystalline silicon nitride powder to a desired level, it is most important to Control the content and distribution of oxygen in the crystalline silicon nitride particles.
However, where the amorphous silicon nitride powder, which is a starting material for the crystalline silicon nitride powder, is produced by thermal decomposition of the imide compound, since it is necessary to remove as much oxygen as possible from the reaction system, it is difficult to greatly vary the oxygen content of the amorphous silicon nitride powder.
This is because the imide compound, which has a high reactivity, easily reacts with oxygen to produce silicon dioxide, and the resultant silicon dioxide remains, as an impurity, in the amorphous silicon nitride powder.